In recent years, devices have come into widely use in which an image is subjected to compression encoding by employing a coding system by handling image information as digital, and at this time compressing the image by orthogonal transform, such as discrete cosine transform or the like, and motion compensation, taking advantage of redundancy which is a feature of the image information, in order to perform highly efficient transmission and storage of information. Examples of this coding system include a Moving Picture Experts Group (MPEG) and so forth.
In particular, MPEG2 (ISO/IEC 13818-2) is defined as a general-purpose image coding system, and is a standard that covers both interlaced scanning images and progressive scanning images as well as standard-resolution images and high-definition images. For example, the MPEG2 is currently widely used in a wide range of applications for professional use and for consumer use. With use of the MPEG2 compression method, in the case of a standard-resolution interlaced scanning image having, for example, 720×480 pixels, an amount of encode (bit rate) of 4 to 8 Mbps is allocated. In addition, with use of the MPEG-2 compression method, in the case of a high-resolution interlaced scanning image having, for example, 1920×1088 pixels, an amount of encode (bit rate) of 18 to 22 Mbps is allocated. Owing to this, it is possible to realize a high compression rate and favorable image quality.
The MPEG2 has been mainly used for high image quality encoding suitable for broadcasting, but has not been compatible with coding systems of an amount of encode (bit rate) lower than that of MPEG1, in other words, a higher compression rate. With the widespread use of mobile terminals, it is expected that the demand for such a coding system will increase in the future, and in response to this, standardization of a MPEG4 coding system has been performed. With regard to an image coding system, the specification thereof was approved as an international standard as ISO/IEC 14496-2 in December 1998.
Further, in recent years, standardization of a standard called H.26L (ITU-T (International Telecommunication Union Telecommunication Standardization Sector) Q6/16 VCEG (Video Coding Expert Group)) has progressed, the object of which was initially image encoding for videoconferencing usage. With H.26L, it has been known that though greater computation amount is requested for encoding and decoding thereof as compared to a conventional coding system such as MPEG2 or MPEG4, higher encoding efficiency is realized. Also, currently, as part of activity of MPEG4, standardization including a function that is not supported by H.26L with this H.26L taken as base to realize higher encoding efficiency has been performed as Joint Model of Enhanced-Compression Video Coding.
As a standardization schedule, an international standard called H.264 and MPEG-4 Part 10 (Advanced Video Coding, hereinafter referred to as AVC) was established in March 2003.
Furthermore, as an extension of the H.264/AVC, standardization of FRExt (Fidelity Range Extension), including encoding tools necessary for business use, such as RGB, 4:2:2, or 4:4:4, as well as 8×8 DCT and quantization matrices defined in MPEG-2, was completed in February 2005. Accordingly, a coding system capable of favorably expressing even film noise included in movies, using H.264/AVC, has been established, which is used for a wide range of applications such as Blu-Ray Discs (registered trademark).
However, there have recently been growing needs for encoding at a higher compression rate, for example, needs for compressing an image having about 4000×2000 pixels, which is four times that of a high-vision image, or for distributing high-vision images in an environment with a limited transmission capacity, such as the Internet. Therefore, in a Video Coding Expert Group (VCEG) under ITU-T, which is described above, studies for improving encoding efficiency have been continuously performed.
Therefore, for the purpose of further improving an encoding efficiency compared with the AVC, standardization of a coding system called High Efficiency Video Coding (HEVC) is currently being conducted by Joint Collaboration Team-Video Coding (JCTVC), which is a joint standardization team of ITU-T and ISO/IEC (see, for example, Non-Patent Document 1).
However, in the HEVC standard disclosed in Non-Patent Document 1, a technique called “Intra Transform Skipping” is employed (see, for example, Non-Patent Document 2).
That is, first, a flag relating whether Transform Skip (referred also to as an “orthogonal transform skip”) is applicable in the sequence, is transmitted to a Sequence Parameter Set (SPS).
When the value is 1, it is possible to apply the TransformSkip to 4×4 luminance and chrominance orthogonal transform blocks.
A flag relating on/off of the TransformSkip is transmitted to each block.
With respect to the block to which the TransformSkip is applied, there is no change in entropy encoding, quantization, loop filter processing or the like.